1. Field of the Invention
The invention relates generally to the field of electronic circuitry and, more particularly, to methods and apparatuses for providing a transition delay matching circuit where a divided clock has substantially the same amplitude and transitions at substantially the same time as a full clock.
2. Background Art
A clock signal is used to synchronize components in a circuit so that each component begins its operation for a given cycle simultaneously. Different clock signals require different frequencies depending upon the application for which the clock signal is used. Clocks in present-day electronic circuits typically must transition much more quickly than clocks in older circuits.
Many electronic circuits utilize multiple clocks. Each clock defines a single clock domain. In some circuits, a clock division circuit is used to divide a reference clock into one or more divided clocks, each defining a separate clock domain. Multiple clocks are used for various integrated circuits including, but not limited to, application specific integrated circuits (ASICs), digital signal processors, microprocessors, and controllers. These integrated circuits are used for a wide variety of applications including, but not limited to, computing, networking, communication, telecommunication, and data transmission. Each of these applications may have one or more high frequency clock domains.
For clock signals with higher frequencies, a shorter duration is available for each transition, and the clock signal may not be able to completely transition from a starting voltage level to a final voltage level. The peak-to-peak signal swing may thus be smaller than the maximum voltage swing permissible by the clock driver. Conversely, clock signals with lower frequencies have a longer time duration available for each transition and the clock signal may be able to completely transition from the starting voltage level to the final voltage level. The peak-to-peak signal swing may thus be greater because more time is available to complete the transition. However, for the next clock transition, the lower frequency clock signal starts from a higher (or lower) voltage level and therefore requires a longer time period to transition to a mid-point (e.g., see FIG. 2A described below). Thus, a skew exists between the transition delay of a high frequency clock signal and the transition delay of a low frequency clock signal where the transition delay is the time required for a clock signal to transition from a final voltage level to a mid-point.
When this skew between the transition delay of the high frequency clock and the transition delay of the low frequency clock is added to the setup time for a receiving device clocked by the high frequency clock, the receiving device may not detect the low frequency clock in the proper high frequency clock cycle. Specifically, if the sum of the skew and the setup time at the receiving device is approximately equal to the period of the high frequency clock and the circuit is under noisy conditions, the receiving device may randomly detect the low frequency clock in either the proper cycle or a cycle too late. More specifically, this condition may occur when the high frequency clock is a harmonic of the low frequency clock.
What is needed is a transition delay matching circuit that matches the transition delay of a low frequency clock to the transition delay of a high frequency clock that is a harmonic of the low frequency clock.
Briefly stated, the present invention is directed to methods and apparatuses for implementing a transition delay matching circuit for which the one or more output clocks of the transition delay matching circuit have substantially equivalent transition delays to bandwidth-limited clocks having a frequency equal to a harmonic of the frequency of the output clocks, and applications thereof.
In an embodiment, the invention operates by limiting the amplitude of the output of the transition delay matching circuit. In a preferred embodiment, the amplitude of the output of the transition delay matching circuit is substantially similar to the amplitude of the reference clock. In the preferred embodiment, the divided clock and the reference clock transition after approximately the same amount of delay because their amplitudes are substantially similar.
In a embodiment of the present invention, the amplitude of the divided clock is limited by placing one or more resistors between a positive divided clock output and a negative divided clock output.
In an alternate embodiment of the present invention, the amplitude of the divided clock is limited by placing a resistor between a divided clock output and a common mode voltage level.
Further embodiments, features, and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.